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Re: [Qemu-devel] [PATCH] blockdev-backup: don't check aio_context too ea

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH] blockdev-backup: don't check aio_context too early
Date: Tue, 7 May 2019 10:50:21 +0200
User-agent: Mutt/1.11.3 (2019-02-01)

Am 06.05.2019 um 22:33 hat John Snow geschrieben:
> in blockdev_backup_prepare, we check to make sure that the target is
> associated with a compatible aio context. However, do_blockdev_backup is
> called later and has some logic to move the target to a compatible
> aio_context. The transaction version will fail certain commands
> needlessly early as a result.
> Allow blockdev_backup_prepare to simply call do_blockdev_backup, which
> will ultimately decide if the contexts are compatible or not.
> Note: the transaction version has always disallowed this operation since
> its initial commit bd8baecd (2014), whereas the version of
> qmp_blockdev_backup at the time, from commit c29c1dd312f, tried to
> enforce the aio_context switch instead. It's not clear, and I can't see
> from the mailing list archives at the time, why the two functions take a
> different approach. It wasn't until later in efd7556708b (2016) that the
> standalone version tried to determine if it could set the context or
> not.
> Reported-by: aihua liang <address@hidden>
> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1683498

Signed-off-by is missing, and a testcase, too. :-)

> diff --git a/blockdev.c b/blockdev.c
> index 79fbac8450..a81d88980c 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -1872,10 +1872,6 @@ static void blockdev_backup_prepare(BlkActionState 
> *common, Error **errp)
>      }
>      aio_context = bdrv_get_aio_context(bs);
> -    if (aio_context != bdrv_get_aio_context(target)) {
> -        error_setg(errp, "Backup between two IO threads is not implemented");
> -        return;
> -    }
>      aio_context_acquire(aio_context);
>      state->bs = bs;

The actual change looks good to me.


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